One of the primary steps in the fabrication of modern semiconductor devices is the formation of metal and dielectric films on a substrate by chemical reaction of gases. Such deposition processes are referred to as chemical vapor deposition or CVD. Conventional thermal CVD processes supply reactive gases to the substrate surface for heat-induced chemical reactions to take place to produce a desired film. The high temperatures at which some thermal CVD processes operate can damage device structures having layers previously formed on the substrate. A preferred method of depositing metal and dielectric films at relatively low temperatures is plasma-enhanced CVD (PECVD) techniques. The PECVD techniques promote excitation and/or disassociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma of highly reactive species. The reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such a PECVD process.
Graded PECVD processes are used to enhance adhesion or other interfacial properties that improve electromagnetic performance and other device qualities. The graded PECVD processes require changing of gas or liquid flows and/or pressures during a deposition process. The standard practice is to optimize the control of each individual parameter, such as the throttle valve setting, to achieve a fixed output, such as pressure. However, if employing a servo pressure control, for example, there is an unstable period as the pressure is brought under control this leads unpredictable and sub-optimal interfacial properties in the graded interface or thin layer. One way of addressing this is to eliminate the servo control and use fixed throttle valve settings during transition steps. Such a fixed position may be chosen by examining what setting produces a stable pressure during a steady state running of the next flow regime. However, this setting may change over time, so that the fixed setting may not be appropriate in future situations. Furthermore, this technique using a fixed throttle valve setting does not allow for any correction to the throttle valve to handle flow or plasma power instabilities that may occur as gas flows change from one setting to the next. Another method employs specifying a linear ramp rate when changing parameters such as power levels. This approach does not account for dynamic conditions and relies upon the correctness of the linear relationship and assumes the conditions do not change.
A graded interface is therefore difficult to attain in PECVD tools operating in their normal mode because the pressure control through the throttle valve and the flow control through the mass flow controller and the power control through the matching network do not perform well under dynamic conditions. Hence, it is generally necessary to stabilize flows and pressure prior to turning on the power and initiating deposition. Once a deposition is started, there is also a certain time before the matching network stabilizes the plasma power. Thus, only a discrete series of layers is attainable. In order to achieve the most repeatable stable film properties, each deposition step is much longer than the time required for the matching network to stabilize the plasma power.